CU(2) MAGNETIC CORRELATIONS IN YBA2CU3OX STUDIED BY MOSSBAUER-SPECTROSCOPY ON SUBSTITUTED YB-170(3+) .1. YB-170(3+) HYPERFINE PARAMETERS (X-CONGRUENT-TO-7.0) AND STATIC PROPERTIES OF THE CU(2) DERIVED MOLECULAR-FIELD (X-CONGRUENT-TO-6.0)

We describe a method, based on the field dependence of the electro-nuclear levels of Yb-170(3+) diluted for yttrium in YBa2Cu3Ox, which probes magnetic fields at the rare earth site. Measurements were made using Yb-170 Mossbauer spectroscopy. When x congruent-to 6.0, we have evidenced an internal (molecular) field on the Yb3+ attributed to the net exchange interaction with the ordered Cu(2) sublattice. The field has an average size of 1600 G and lies at an average angle of near 45-degrees to the c-axis. Up to a least 60 K it remains static on the frequency scale of the experiment (5 x 10(8) s-1). Some possible origins of this previously unsuspected field are discussed.